Cyclic fluid pressure overlapped multicontact switching mechanism for rectifiers



J. A. FAV 2,616,997 FLUID PRESSURE MULTICONTACT 4 TIF'IERS Nov. 4, 1952 CYCLIC OVER PED v SWITCHING MECHANISM FOR REC Filed July 18, 1951 3 Sheets-Sheet l nw w,

Nov. 4, 1952 J. A. FAVRE CYCLIC FLUID PRESSURE OVERLAPPED MULTICONTACT SWITCHING MECHANISM FOR RECTIFIERS 3 Sheec s-Sheet 2 Filed July 18, 1951 Fig.4.

CONS Til/Y7 PRESS URE SOURCE VARIABLE SGURCE Inventor: John A.Fa-vre,

His Attorney.

John Afl avre,

Hus Attorney.

Patented Nov. 4, 1952 NI TED STATES .0 F F I CE .CYCLIC FLUID PRESSURE OVERLAPPED SWITCHING MECH- LANISM FiOR [RECTIFIERS 'John'A.'Favre,Drexel Hil1,' Pa.,5'assignor to Gen- ;eral ElectricCompany, a corporation of New MU LTICONTACT York AppIicatiOn'JuIy'IS, 1951, Serial No. 237',379

(Cl. ZOO- .81.4)

:9 Claims.

The invention" relates to cyclic overlap switching mechanism, particularly of "a high speed -multi-contact type adapted for synchronized operation 1 inconnection with suitable electrical circuits and apparatus for converting polyphase "alternating currents of commercial frequencies todirect current. or vice versa.

In -'ccmmercial alternating current rectifying service, particularly in the relatively low and medium -voltage ranges such as' ordinarily used in electrolytic work and the like a mechanical rectifierswitch -mechanism provides the highest overall operating efiiciency duetothenegligibly low voltage drop through the switching-contacts.

'But to'obtain this advantage it becomes essential that precisely synchronized over-lapping closure and: opening of aseries of'switching'contactscontrolling the rectifying circuits and apparatus be obtained-in extremely rapid recurrent cycles'in order properly 'to effect the rectifying action'by sequentially transferring the current conduction from each rectifying circuit to another at Ithe ing'eccentric cam -or wobble'plate sliding .pllsh 'rod' operated'rectifier switching mechanisms have beenproposed in an endeavor to meet .the very exacting high speed cyclic overlap switching .requirements encountered in commercial alternating current-rectification service. '-But.such-mechanical .sliding push rod operated rectifier switching mechanisms even though welllubricated inherently ,pro'duce appreciable wear of the operating parts or .the .mechanism at the required frequencies of .contact operation since withthe usual.6,0cyclealternating current over, 5,000,000. closures and openings of each. rectifying switch .contact must be effected each day a of operation. .This may soon reach .the usual life expectancy of mostlany mechanical sliding-push .rodcperated switchingdevice capable. of rectifying currents of substantialvalue.

.Also .with .such'eccentric or \wobble palate; and sliding pushrod operatedrectiiyingswitch mechanisms itusually is-a rather-tedious and difficult task to accurately adjust :the {relative lengths ofthe several contactpush=rodsisozas to secure the precise overlapping ,CIOSU'IBSYQT the-contacts required :for uniform contact=zclosure or dwell. as

well as to readjust-when itbecomes .necessaryto compensate for any unequal wear .and erosion of theecontacts: aswell asrfor any uniform .variation :of the gd-well .and consequently the -ovei'1ap of :allthe contactswhenrequired toobtain-proper commutation upon *a substantial variation of the load condition since only very minute lineal variations-in the relative lengths of thepush rods are involved-per degree of rotation of the actuating -eccentric or Wobble plate.

One "of the objects of the present invention is to provide an improved fluid pressure synchronized cyclic overlap rectifier contact operating mechanism having 'an improved permanently fixed and precisely angularly displaced -multiport rotaryvalvefor efiecting substantially isochronous cyclic overlapping operation of all the rectii'ier switching contactsand thereby obtain 'not I only accurately synchronized and uniformly overlappin dwell thereof during the rapidlyrepeated cycles-but-also-provide a materially increased life of the rectifier contact synchronizing mechanism particularly when oil having good 'lubricating characteristics is employedas the fluid pressure synchronizing medium. A further important-technical advantage-is that any inherent wear and erosion of the-contactsor other parts is automatically-minimized by means of -the improved rotary multiport valve fluid pressuresynchronized contact operating mecha- -nism=withoutresulting in any'adverse variations in the overlap or dwell of the contacts.

- Incarrying out "the present invention in a preferredform, aseparate fluid pressure-responsive operaton'preferably inthe form of 'a relatively small and light-weightpiston, is provided for'operating each ofa series of rectifier contacts and the fluid pressure applied to the series'o'i .pistons is cyclically varied in overlapping synchronism thereby to overlap the closures and openingso'f the rectifying contacts in the series :bymeans of a synchronously driven rotary valve :having a cooperating valve body provided with permanently fixed and precisely angularly displaced pressure control ports communicating With the -.operating pistons to open the next preceding rectified .contact "in the series before elapsed-beforethe contact closes will 'alwaysconform with the standard accelerationnquation and the provision "of'a'uniform mass for each piston and contact and 'a uniform accelerating pressure applied thereto will inherently .tend to :give .isochronism -or--constant timing. .lMoreover,

any small variations in "the-travel iof the sequentially= overlapping contacts due to "any inherent wear or erosion of the parts become relatively inconsequential as the elapsed time required to travel a given distance varies directly with the velocity, which at the end of the stroke is relatively high. Thus the present invention is able to utilize a square root function instead of merely a linear one and consequently the total time becomes relatively stabl and effected to only a rather negligible degree by small variations in the contact travel due to any inherent wear or erosion of the parts. This is also true of any overlapping dwell thereof during the rapidly revagrant or incidental changes in fluid pressure applied to the pistons.

Another object is to provide for uniformly varying the dwell of all the overlapping rectifier contacts by substantially varying the pressure at which the fluid synchronizing medium is applied to the contact operating pistons by means of the rotary valve. Thus, a desired change in the duration of the contact dwell can be readily accomplished by changing the fluid pressure applied to the piston to a somewhat greater degree than the incidental relatively small changes due to abnormal operating conditions such as changes of viscosity with temperature, etc. When the fluid pressure is increased in order to increase the duration of contact dwell, a higher velocity at the time of contact closure will result and hence the increased kinetic energy must be properly absorbed in order to avoid increased contact wear or erosion. But by providing a special mounting for each switch contact having suitable resilient characteristics, the force of impact of the moving contact can be suitably controlled so as to obtain reasonably long contact life. Furthermore, the variations in the contact dwell by materially increasing or decreasing the applied pressure can increase or decrease the dwell by earlier or later closure of the contacts but, if desired, may be specially arranged to lengthen the dwell at both ends. However, where a variation in the dwell at both ends becomes necessary, this may be obtained most simply by a relative angular displacement of the body with respect to the rotary valve to obtain earlier or later opening of the contacts in conjunction with an effective variation in the fluid pressure applied to the pistons to obtain the earlier or later closure of the contacts.

Further objects and advantages of the invention will appear in the description of the accompanying drawings in which Fig. 1 is a top view, partly in section, of a cyclic fluid pressure overlapped six contact rectifier switching mechanism embodying the invention; Fig. 2 is an end view partly in section showing a symmetrical radial mounting of the six operating pistons and switch units around the fluid pressure cycling rotary valve element; Fig. 3 is a cross-sectional view on the line 3-3 in Fig. 1, showing details of the fluid seal and pressure outlet construction of the rotary valve mechanism; Fig. 4 is a cross-sectional view of a modified form of cyclic fluid pressure overlapped rectifier switching mechanism having two aligned rows of three contact operating pistons with each pair mounted in opposing alignment in the cylinder block to communicate on opposite sides of the rotary fluid pressure cycling valve element having fluid pressure cycling ports angularly displaced therein; Figs. 5, 6, and 7 are cross-sectional views respectively on the section lines 5-5, 6-6, and 1-1 in Fig. 4, showing the structural details of the aligned pairs of pistons and contacts as well as the angular displacement of the pressure controlling ports in the rotary valve element; Fig. 8 is a cross-sectional view of a further modified form of the invention wherein the six rectifier contacts and the operating pistons therefor are mounted in linear alignment in a single row in the cylinder block; and Figs, 9, 10, l1, 12, 13, and 14 are respectively cross-sectional views showing the angular displacement of the pressure cycling ports provided in the elongated rotary valve element for cooperating with the respective rectifier contact operating pistons. Fig. 15 is a cross-sectional View of a modified bridging contact structure showing a separately adjustable stop to effect individual adjustment of limit of the contact opening travel.

In the radial form of rectifier six contact switching mechanism shown in Fig. 1, the combined cylinder block and valve body It) preferably is of hexagonal cross-section as shown in Fig. 2 and provided with a radial cylinder extending to each face for mounting therein a corresponding one of the series of pistons Ila, b, c, d, c, ,1. With this construction the inner end of each radial cylinder communicates directly with the angularly displaced ports [2a and |2b formed in the central fluid pressure cycling rotary valve element E2. Each piston operates a corresponding one of the series of rectifier switch units i3a, b, c, d, e, f, that are removably mounted on a corresponding face of the hexagonal cyclinder block H) by suitable holding screws I4 as shown in Fig. 2.

The switch unit lI-ia, as shown in the crosssection, is provided with an insulating body I5 for supporting the alternating current conductor l6 and the direct current conductor I! in spacedapart alignment with the gap therebetween symmetrical with the axis of the corresponding cylinder. Each conductor 16 and I! has a stationary contact 28 removably mounted thereon by means of the screw 2! and provided with a contact facing 22 formed of suitable contact material for withstanding long-continued heavy duty service. The cooperating movable bridging contact 23 is formed of similar material and is carried on an insulating mounting block 25 formed to slide in the cylinder and provided with an end extension for engaging with a resilient shock-absorbing buiier 26 carried inside the hollow piston Ha.

The movable bridging contact 23 may be removably mounted on the insulating block 25 by means of the pin 28 extending through contact 23 from the insulating push rod 29 into a suitable guide opening formed in the face of the block 25. A pin 3! aligns the other end of the push rod 29 in the guide member 3| that receives the contact opening thrust of the compression biasing spring 32. The contact opening biasing thrust can be varied by varying the position of the adjustable screw plug 33, relative to the insulating body 15. In this way the push rod 29 is maintained in axial alignment with the cylinder and piston I la, as well as the opening biasing spring 32 so that the movable contact 23 is readily operable in self-aligning engagement with the pair of spaced-apart stationary contact faces 22 when fluid pressure is cyclically applied to the inner face of the piston I la.

Each of the other switch units [3b, 0, d, e, f is of the same construction as the switch unit I3a and for three-phase double-wave rectification, the alternating current conductors N5 of the diametrically opposite switch units Na and HM may be connected together to one phase, the alternating current conductors of the diametrically; opposite units Nb and I-3e may be connected. togetherito another phase, and the alternating currentconductcrs of thediametrically opposite units'l3c .-.and:I'3j maybe connectedtogether to the third .phase. ternate units. I3a, E30, and I3e maybe connected together to; oneside of the direct current circuit, 'Whilethe directcurrent conductors of the alter- .nate units I312, I3d, and 3f may be connected The :direct current conductors of the altogetherto the othergside' of the direct current circuit. .=Any desired rectifier circuits with suit- ;able'. transformer. and *commutating control apparatus can be used, for example, suchas shown in the Koppelrnan; Patent 2,341,326.

:In order cyclically and sequentially to vary" the gfluid pressure applied tothe, pistons I la; b, c,,d, e, 1, the'rotary valve element I2 is rotatably mounted -in-.the cylinder blockuand valve body ID with its axis located at the point-of intersection of the axesof the diametrically Opposite pairs; of radial cylinders-formed 1 in the'block ID. The rotary 'valve'element I2 may be provided with an operatving shaft 40 that can readily be connected to be continuously driven at synchronous speedby a suitable electric motor energized from the three- ,phase alternating current circuit. The :ball tbeari' ng 4I-may-be-provided forrotatably supporting the shaft 10 so as to axially ,align the valve member I2-in'-the block I when the end plate-42 is secured in place by the bolts 43. Also suitable oil'sealing labyrinth rings 44 may be provided for fluid-tight sealing of the driving end ofthe valve while the endplate'45 seals the other end-of the valve body I0.

-Preferably oil of good lubricating quality, ,or if desired, other suitable liquid or gas medium may be suppliedtothe-rotary valve-body I0 :under pressure through-the inlet conduit- B-that communicates with the inner annular conduit -5I shown as formed jointly in the rotary valve member I2 and the body I Ilso astocontinuously supply the fluid under pressure through the radial passage I20 to the axial passage IZd,-and'thence ,to the continuously driven rotary pressure distributing valve port I 2a. 'The end of the axial passage I2d-maybe-sealed by a screw; plug I2e.

Upon synchronous-rotation of the rotary'valve element I2, the-radial-valve-port I2a is carried into: communication successively with the inner ends of the radial cylinders forthe pistons [2a, b, -c, cZ,-e, 1, so as to successivelyapply fluid pressure thereto and ,therebyaccelerate and close the switches operated bythe pistons in a predetermined I overlapping sequence. The operating fluid pressure is maintained applied to each successive piston until the exhaust valve port I2b that is formed in the rotary valvemember I2 in the desired predetermined angular relationship with the pressure inlet'port I2a is successively brought into communication with each radial cylinder in the block I0 upon the continuous'rotation of the valve member I2. Subsequently, the oil pressure, in the; cylinder isvented through the exhaust port" I2b as indicated in Fig.- 1 into an annular oil passageway 52, to which the oil outlet or returnconduit-53 is connected as shown more clearly in the cross-sectional view. of Fig. 3.

oil leakage pastthe piston is :drained into :.the return pas ageway 52 by means of an annular passageway 55 formed in each cylinder near the outer. end of the pistonand interconnected by a drain passage 55 .with the annular oil return -pa"sage;i52.

Thus upn operation in alternating. current :rectifier service; the valvemember; i2: is continuously rotated'at synchronousspeed togapplyrflu-id :pressure successively; to the series dimistonsaas the, oil. pressure port I 2a is successively brought into communication therewith at; each .60 ;:an 1- -:lar amovement lot .the "valve element I2. :Each piston .will respond: to accelerate and close :the

corresponding switch. iWith ithe. predetermined zangular relationshipof the .inletport I'Zaand the exhaust, port I2b as shown, 1 the oil; pressure -,is.maintained appliedto eachpiston untilgthe continued, rotation; of valve member: I 2 through .approximately 1510" :brings the: exhaust port I2b into communication therewith. :Thereupon,;.the oil pressure will. be exhausted and the biasin spring-becomes effective to. return the contacfrto the open. position. .The, communication ofxeach cylinder with: the exhaust; port I I Zbmaycontinue "for substantially of the'valve rotation.

.cUpon rotation of the valve element, I2 from the However,

cemmunication with the ,piston T I In to exhaust the pressuretherefrom and effect the open-ingyof :the corresponding-eontacts,1-before theoil :pressure port 1I-2a :is brought into communication .with'. piston II). In this way;.the predetermined :angular relationship-of ;the pressure applying sport I;2a; and the pressure exhausting; port I 2b insures a predetermined overlapped closure 5 of the next succeeding. switchtinl the: series and: the

opening of eachpreceding switch duringthe .closure of each switch. ;Thus, the switch oper- "ated bycpiston l I Ie .is closed before the switch ,.operated by .piston ;I I0 ,is gopened during the .closure of the switch operated by piston I Id.

The sequential operation, of the other-contacts in the series will be accomplished in, exactly the same manner so that the closureofeach switch i'preciselyoverlaps. the; closure ofv the, next precedqing switch in. the seriesv as well as. the. nextsucceeding switch in the series, as :required to produce; proper commutation ofnthe three phasaal- .ternating; current. Thus, as shown in Fig.2; the

closure/of contacts-I Ic and-IId is -overlapping,

sand during::the.next;30 .of angularrotation of the-valve I2, the closure of. contact IIe:,also becomes overlapping. But after such;30yofangular rotation, contact I I c ,is opened; so that. only contacts Nd and He remain overlapping until the next 30.r0tation of valve- I-2 producesoverlappingclosure ofcontact II f. Thesuccessive precise overlapping closure of the other contacts in" the series is accomplished -in exac ly the; same way.

Due to q the fluid presure synchronization ;of the overlapping operation: of the switches; automatic compensation is obtained for any contact weary and erosion without resulting in anyuadverse-variations in the-overlap or 'dw'ell.;in.the

cyclic closures of the contacts, thus .enabling accurately synchronized commutation of thereo- 1.tifiedcurrent to be obtained throughout .a relatively long period of rectifying switching service,

particularly when oil is .used as the fluid pressure synchronizing medium.

The oil. under pressure may besupplied to the :rotaryvalve I2 throughconduitxfiil frompany suitable variable pressure source 54, such for example as a continuously driven gear pump drawing oil from the drain return sump and provided with an adjustable pressure responsive by-pass valve for varying the oil pressure supplied to conduit 58. Thus, when it is desired to vary the closure or dwell of the movable contacts of the rectifier switches, all that is necessary is to vary the pressure at which the oil is supplied to the rotary valve i2. When the oil pressure is decreased, the contact acceleration will be decreased and hence the initial closure of each contact will be correspondingly delayed, and when the oil pressure is increased, the con tact acceleration will be increased and hence the initial closure of each contact will be advanced. In case it should be desired to change the termination of the contact closure relative to phase, then the valve body Ill may be rotated relative to the valve member l2 (or the motor may have a stator shifter) so as to correspondingly retard or advance the communication of the exhaust port |2b with each of the successive cylinders. Consequently, by both varying the oil pressure to retard or advance the initial closure and rotating the valve body In to advance or retard the termination of the closure, variations at both ends of the dwell of the contacts can be readily obtained as required to obtain proper commutation under varying load conditions, or to control the voltage of the rectified direct current.

As indicated in Fig. 15, if desired, each of the movable bridging contacts 23 that are biased to the open position in Fig. l as well as the movable bridging contacts in each of the modifications may be provided with a suitable separately adjustable stop I that may be screw threaded in the body It! as shown to effect individual adjustment of the limit of contact opening travel and thereby individually control the contact timing to obtain uniform dwell in case of any.

manufacturing or other variations in the parts.

In the modification shown in Fig. 4, three cooperating pairs of pistons and contacts are mounted in double row alignment in an elongated cylinder block and valve body I801. In this modification, the contact biasing springs are eliminated and the biasing of the pistons to open the contacts is accomplished by a fluid pressure admitted from source 65 through the conduit 65 to the passageway 61 that communicates through suitable cross-passages 68 as shown in Figs. 5, 6, and '7 with the space in the cylinder above each of the pistons 19a, b, c, d, e, 1 that operate the corresponding movable contacts Ha, b, c, d, e, f of the rectifier switching mechanism, so as normally to bias each of these pistons and contacts to the circuit-opening position. The operating fluid pressure on the opposite or lower side of each of the pistons is supplied from the variable pressure source 89 and cyclically varied by means of the elongated rotary valve member 12 that is synchronously driven and may be provided with a suitable seal such as shown in Fig. 1.

The piston 38a is operatively connected to the rectifier contact Ha by means of the insulating push rod 12a that is slidably mounted in the guide plug i311 that is suitably secured in the upper end of the cylinder in which the piston Hm operates. The other pistons and contacts are operatively connected in a similar manner. The pair of rectifier contacts Na and Nb serve to interconnect the alternating current phase conductor #5 to either of the direct current conductors T6 or ll. In a similar way, the pairs of contacts He and lid and He and Hi serve to connect the other alternating current phase conductors i8 and '19 respectively to either one of the direct current conductors l6 and 11;

In operation, when fluid pressure is supplied from the variable pressure source 69, through conduit '59 to the axial passageway in the rotary valve element 12, this pressure will be communicated through the radial passageway iii to the port 82 and thence to the underside of the piston 70a as shown in Fig. 5. As a result, the pressure differential on piston 18a becomes effective to accelerate and operate the bridging contact Ha into circuit closing engagement with the conductors 15 and 16. Upon continued rotation of the valve element 12 through approximately 150 degrees, the exhaust port 83 will communicate with the underside of piston 10a to exhaust the fluid pressure therefrom through the annular passageway 85 formed in the valve body Illa so as to exhaust the fluid pressure through the outlet conduit 86 and thereby permit the biasing fluid pressure on the upper side of piston 58a to return the contact Ha to the circuit opening position.

Both contacts Ha and. Nb will remain in the circuit opening position until a further rotation of valve element 12 through approximately 30 degrees, thereupon the pressure inlet port 82 will communicate with the underside of piston 10b to effect acceleration and operation of the contact 'Ilb to interconnect the alternating current conductor 15 with the direct current conductor 11. Contact 'Hb will be maintained closed through substantially 150 degrees angular rotation of the valve element 12, until the exhaust port 83 is brought into communication with the underside of the piston 10b to exhaust the pressure therefrom and return contact Ilb to the open position. Both contacts Ha and Nb again will remain open until the succeeding angular rotation of valve element 12 again brings the pressure inlet port 82 into communication with the underside of piston 78a to close contact Ha.

As shown in Fig. 6, the rotary valveelement I2 is provided with the axially aligned inlet pressure port 82a and exhaust port 83a for cyclically varying the pressure on the underside of the pistons 10c and 79d in exactly the same way as just described, except that these ports are angularly displaced degrees from the ports 82 and 83, in order to produce the precise overlapping operation of the rectifier contacts as required to produce the proper commutation of the alternating current. Similarly, the corresponding pressure controlling ports 32b and 831) are angularly displaced another 120 degrees and axially aligned for effecting similar sequential operation of the rectifier contacts He and Hf to produce the required precise overlapping closures thereof in synchronism with the reversal of the alternating current in order to produce the desired rectifying action.

Upon substantial variation of the pressures applied to the inlet conduit 19, the contact acceleration can be either increased or decreased, and hence the closure of each contact will be correspondingly advanced or delayed, in order to uniformly vary the dwell of each of the rectifier contacts. By effecting an angular displacement of the rotary valve member 12 with respect to the synchronism of the alternating current, and also varying the oil pressure, variations at both ends of the dwell of the contacts can be readily obtained as required to obtain proper communicaaci -ear 9. tion 'undenvarying "loadconditions or to control the voltage of the rectifier" direct' current;

In'the further modification shownin Figs8, all

six of thepistons 93ab-c-d'e--f and rectifier switching contacts I 04a-b'-c-d-ef operated thereby are mounted in alignment in a single row in a further elongatedicylinder: block. and

valve body 90, having the correspondingly elon-' gated rotary valve elementi lil that may be operated synchronously-therein in the same manneras previously described; Fluid pressureis sup-- plied from a suitable variable-pressure source. 95 through the conduit 96- to the axial passage 9T formed in the rotary valve-element 91 Toactuate the piston 93a, the fluidpressure is communicated from the. axial passage 91. through the radial passage 98tz-to the port I that.wi1l

upon rotation of'valve BI-in-thedirection indicated by the'arrow' in Fig. 9 register with the port- IOI formed in the valve-body 90 50 as-tocommun-icate thepressure to the underside of the piston 93a. The resulting thrust of 'piston'93a moves the piston rod I02 to operate the rectifier movable contact member I040, that may be removably mounted on the insulating block- M3 by means of the insulated'mounting bolt I05.

In this way the contact member I04 is accelerated and carried from the 'open position to which'it previously hasbeen operated by fluid pressure into circuit-closing engagement with the station ary switch contact IOSa-and" I09Zz;

As indicated in Fig. 9, when the-angular rota-- tion of the valve element!!! brings the port' I80" into register'wlth the port IOI so as tocommunicate the-contact closing pressure to" the underside of the piston 93a, fluid-pressure can be exhausted from the upper side of the piston 930 by the registering of the exhaustport I I I with the port H2 formed formedin-thebody SO and' communicating through the passage I l3 with=thecylinder-above the piston 93a; This-exhaust port- III communicates through the lateral passage H4 formed in the periphery ofthe-rotary valve element 9i with the annual grooveI I 5 formed in the body 90 and thence is discharged through the conduit II6 ,to a suitable sump from which the fluid may be returned to the variable pressure source 95 in a suitable manner as previously described.

Each of the successive pistons 93b-cde-f Y in the series is operated in the same way as the piston 93a, except that the inlet ports I i30bc def and the exhuast ports IIIbcef are successively angularly displaced from each other by 60 degrees as indicated in the crosssectional views of Figs. 10, l1, l2, l3 and 14. Consequently, the series of pistons 93abc-- def are operated in the predetermined sequence required to sheet the cyclic overlapping closure and opening of the series of contacts I04abc-def in sequence so as to elTect proper commutation and rectification of 3-phase alternating currents. To this end, the switch contacts Illfia and IOSb may be connected to one phase conductor, the contacts Iilla and IIl'Ib to the second phase conductor, and contacts I08c and I08b to the third phase conductor of the alternating current circuit which may be supplied from any suitable source such as the secondary of a Y-connected transformer. Likewise, the switch contacts IIJEJa-b-c may be connected to one side of the direct current circuit and the switch contacts IIIla-bc connected to the other side of the direct current circuit.

As indicated in Fig. 8, the annular grooves I I5a-b'c and separate drains II 6a--b-c maybe located b'etween each adjacent-pair of the cylinders 93abcej so as to communicate with the lateral-passages I|4a'-bc that interconnect the adjacent pairs oi exhaust ports- I I I abce'j or if desired, all of the passages II4ab"-c may be interconnected together to drain into one common drain-conduitI I6 While I have shown and described certain par ticular embodiments ofimyinvention, I do not desiremyinvention to be limited to the constructions-shown and described for 'it 'will, of course, be obvious that changes may be made-without departing from-my invention. I 'thereforeaim in the appended claims to cover all such changes and modifications as fall" within the truespirit and: scope of my inventions What I-claim as new and-desire-to obtain by Letters Patent of the United States is:

1. A cyclicoverlap alternating currentrectifier switching mechanism 2 having in combinatiorr a series of switches, each-havingaseparatefluid pressure responsive operator,' and means for cyclically varying the-fiuid pressure applied to the operators to sequentially close'and-open the series of switches in recurrent cycles'includinga synchronously driven multi -port rotary valve mechanism' provided with fluid pressure control passages communicating with the operators to close the next succeeding switchin :the series'before opening each switch.

2. A cyclicoverlapalternating tcurrentrectifier switching mechanism having; in. combination a' series of switches, each havingaseparateflirid pressure responsive operator; andmeans'for cy clically varying the fluidpressure applied'to'the operators to sequentially close'andopen the'series of switches in recurrent cycles including 'a syne chronously driven multi-port 'rotary valve mechanism provided with fluid pressurecontrolpas sages communicating with'the operatorsrto'close the next succeeding switch'in'the series before opening each switch, andmeansforcontrolling the fluid pressure-supplied"totthe valvemecha- 'nism. to vary the duration of closure of the switches.

3; A cyclic overlap polyphase-alternating current rectifier switching mechanism having in combination a series of switches, each having a movable bridging contact, a separate fluid pressure responsive operator for each bridging contact, and means for cyclically varying the fluid pressure applied to the operators to sequentially close and open the bridging contacts of the series of switches in recurrent cycles including a continuously driven rotary multi-port fluid pressure inlet and exhaust valve element having a cooperating valve body provided with a series of angularly displaced registering fluid pressure control passages communicating with the operators to close the next succeeding bridging contact in the cycle before opening the next preceding bridging contact in the cycle during the closure of each contact.

4. A cyclic overlap polyphase alternating current rectifier switching mechanism having in combination a series of switches, each having a movable bridging contact, a separate fluid pressure responsive operator for each bridging contact, and means for cyclically varying the fluid pressure applied to the operators to sequentially close and open the bridging contacts of the series of switches in recurrent cycles including a continuously driven rotary multi-port fluid pressure inlet and exhaust valve element having a cooperating valve body provided with a series of angularly displaced registering fluid pressure control passages communicating with the operators to close the next succeeding bridging contact in the cycle before opening the next preceding bridging contact in the cycle during the closure of each contact, and means for varying the pressure of the fluid supplied to the valve element to vary the duration of closure of the switches.

5. A cyclic overlap polyphase alternating current rectifier switching mechanism having in combination a series of separately operable switches, each having a fluid pressure operating piston, a cylinder block for the pistons provided with ports in the block communicating with the pistons, and means including a synchronously driven rotary fluid pressure valve element mounted in the block and provided with ports for registering with the ports in the block to effect the closure of the next succeeding switch in the cycle before opening each preceding switch during the closure of each switch.

6. A cyclic overlap polyphase alternating current rectifier switching mechanism having in combination a series of separately operable switches, each having a fluid pressure operating piston, a cylinder block for the pistons provided with ports in the block communicating with the pistons and having separately adjustable means for varying one limit of travel of each piston, and means including a synchronously driven rotary fluid pressure valve element mounted in the block and provided with ports for registering with the ports in the block to effect the closure of the next succeeding switch in the cycle before opening each preceding switch during the closure of each switch, and means for controlling the fluid pressure supplied to the rotary fluid pressure valve element to uniformly vary the duration of closure of the switches.

'7. A cyclic overlap polyphase alternating current rectifier switching mechanism having in combination a plural cylinder block provided with a piston in each cylinder and ports communicating with the cylinders, a plurality of switches, each having a movable contact operated by a corresponding piston and means for cyclically varying the fluid pressure applied to the pistons to sequentially close and open the switches in recurrent cycles including a synchronously driven rotary multi-port valve element mounted in the cylinder block to register with the ports in the block communicating with the cylinders to close the next succeeding switch in the cycle before opening each switch.

8. A cyclic overlap polyphase alternating current rectifier switching mechanism having in combination a plural cylinder block provided with ports communicating with the cylinders and having a piston in each cylinder provided with mean; for biasing the piston to one position, a plurality of switches, each having a bridging contact carried by a corresponding piston and means for cyclically varying the fluid pressure applied to the pistons to sequentially close and open the switches in a cycle including a synchronously driven rotary multi-port fluid pressure inlet and exhaust valve element mounted in the cylinder block to register with the ports in the block communicating with the pistons to close the next succeeding switch in the cycle before opening the preceding switch during the closure of each switch.

9. A cyclic overlap polyphase alternating current rectifier switching mechanism having in combination a cylinder block having a series of cylinders formed therein and each provided with a piston and a control port communicating with one side of the piston, means including passages in the block for supplying fluid pressure to the other side of each piston for biasing the piston to a predetermined position in the cylinder, a multi-port fluid pressure supply and exhaust valve rotor mounted in the cylinder block for register- ;ing with the control ports in the cylinder block 'for varying the fluid pressure differential to op- No references cited, 

